Terra sigillata is a term with at least three distinct meanings: as a description of medieval medicinal earth; in archaeology, as a general term for some of the fine red Ancient Roman pottery with glossy surface slips made in specific areas of the Roman Empire; and more recently, as a description of a contemporary studio pottery technique supposedly inspired by ancient pottery. Usually roughly translated as 'sealed earth', the meaning of 'terra sigillata' is 'clay bearing little images' (Latin sigilla), not 'clay with a sealed (impervious) surface'. The archaeological term is applied, however, to plain-surfaced pots as well as those decorated with figures in relief.

Terra sigillata as an archaeological term refers chiefly to a specific type of plain and decorated tableware made in Italy and in Gaul (France and the Rhineland) during the Roman Empire. These vessels have glossy surface slips ranging from a soft lustre to a brilliant glaze-like shine, in a characteristic colour range from pale orange to bright red; they were produced in standard shapes and sizes and were manufactured on an industrial scale and widely exported. The sigillata industries grew up in areas where there were existing traditions of pottery manufacture, and where the clay deposits proved suitable. The products of the Italian workshops are also known as Aretine ware from Arezzo and have been collected and admired since the Renaissance. The wares made in the Gaulish factories are often referred to by English-speaking archaeologists as samian ware. Closely related pottery fabrics made in the North African and Eastern provinces of the Roman Empire are not usually referred to as terra sigillata, but by more specific names, e.g. African red slip wares. All these types of pottery are significant for archaeologists: they can often be closely dated, and their distribution casts light on aspects of the ancient Roman economy.

Modern "terra sig" should be clearly distinguished from the close reproductions of Roman wares made by some potters deliberately recreating and using the Roman methods. The finish called 'terra sigillata' by studio potters can be made from moist clay, mixed as a very thin liquid slip and settled to separate out only the finest particles to be used as terra sigillata. When applied to unfired clay surfaces, "terra sig" can be polished with a soft cloth or brush to achieve a shine ranging from a smooth silky lustre to a high gloss. The surface of ancient terra sigillata vessels did not require this burnishing or polishing. Burnishing was a technique used on some wares in the Roman period, but terra sigillata was not one of them. The polished surface can only be retained if fired within the low-fire range and will lose its shine if fired higher, but can still display an appealing silky quality.